Bug: GHC creates incorrect code which throws <<loop>>

Yongqian Li yongqli at kerrmetric.com
Wed Apr 1 01:48:32 UTC 2015

Hi all,

I wanted to bring to your attention this bug,
https://ghc.haskell.org/trac/ghc/ticket/10218 , which I filed about
two days ago but hasn't gotten any replies to yet.

In my opinion, this is a rather serious bug since GHC is producing
incorrect code. While there is a workaround, it makes my program take
about 2 hours to compile.

Since reporting the bug, I've also seen "strange closure type"
exceptions, which I believe are also caused by this bug, although I do
not have a test case. Based on my limited understanding of GHC
internals, I suspect the problem lies in how type class dictionaries
are looked up at run time, as the problem disappears if GHC can fully
resolve the types.

Can others see if they can reproduce this bug? If there is any way I
can help, please let me know.

Yongqian Li

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